The PRI vs SIP debate continues…

Apr 27, 2016

In order to choose a solution that is right for you, first you need to have an understanding of how each solution works and knowing each you can make an educated choice.

A good analogy for comparing SIP with a PRI is the difference between owning your own car and taking public transit.  When you own your own car (SIP) you have greater flexibility and features available to you, but it also requires your time to learn how to drive and money for gas and maintenance.  Public Transit (PRI) may not offer this flexibility but the costs of ownership are passed on.  Each solution will take you to your final destination, it is only a matter of deciding what fits your requirements best.


PRI (Primary Rate Interface) are 24-circuit switched digital voice channels with 23 of these channels available for voice traffic (1 is held back for control information), transmitted over the Public Switched Telephone Network (PSTN).
SIP (Session Initiation Protocol) are voice packets transmitted over the internet (riding over the existing Data Network).  SIP connections offer end users flexibility and features not available with a PRI.


Both connections are digital (no analog lines), the difference being in the delivery.  Digital circuit with physical connection routing along a dedicated line to the PSTN vs packet switching with a virtual connection via the existing Data Network then routed to a service provider who then routes those packets to their destination via an internet connection to a local PSTN.

PRI: circuits require a direct physical connection and physical termination hardware and require a voice switch to connect to your phone system.

SIP: circuits utilize your current Data Network to transmit Voice packets and as a matter of Best Practice, should be used with a Session Border Controller (SBC) to ensure your lines are kept secure.


SIP is scalable and can offer automatic “Burst” capability. When SIP shares the data connection being correctly configured for adequate bandwidth and QoS becomes very important to maintain voice quality.

PRI Channels can be scaled through the Telco Company, and if you have a partial PRI this can be very quick and easy, if an additional PRI is required an additional circuit will need to be ordered and installed (as they are a physical connection) and can take time to implement.

Quality of Service (QoS)

With a PRI you receive dedicated Channels that transmit each call directly though the local PSTN.  A PRI can provide excellent QoS and offer clients a Service Level Agreement that guarantee the allowable downtime or reliability of your service.

SIP on the other hand due to the method of delivery can be subject to “Best Effort” services, unless you procure a dedicated data circuit from the SIP provider.  The end user is normally responsible for firmware and device configuration maintenance.  This in turn requires more technical resources and time to manage SIP circuits.


One last item to note, unlike PRI’s that are dedicated lines to the location PSTN.  SIP uses the Public Internet to transmit voice packets.

With SIP lines, a Best Practice to ensure your connectivity is secure you will want to deploy a Session Boarder Controller (SBC).  For example, Ingate’s full SIP proxy technology, which allows for advanced filtering, verification, authentication and routing, as well as dynamic control of the opening and closing of media ports. They also encrypt the SIP signaling using Transport Layer Security (TLS) and media (voice, video, etc.) using Secure RTP (SRTP). With encryption, sessions are kept private with no chance of eavesdropping.

Having multiple choices for your connectivity is wonderful.  Selecting the right choice for you takes a bit of time to ensure you understand what all the Pro’s and Con’s are before taking the leap.

Dyne Day, Glenbriar Technologies Inc.  Service Desk Supervisor and Marketing Associate